Long-term exposure to high levels of noise may cause hearing loss and other health hazards, and as a consequence U.S. law requires that the exposure level to noise by an individual in a work place be accurately measured and limited. See e.g. 29 C.F.R. §1910.95, and U.S. Pat. No. 6,456,199 for a general discussion. Noise dosimeter is commonly used in the industry to measure the cumulative noise exposure by an individual over the course of a full work shift, and the effectiveness of hearing protection devices (HPD) or noise control devices.
U.S. Pat. No. 6,456,199, incorporated herein by reference, discloses a cost-effective and unobtrusive means of continuously monitoring an individual's actual noise exposure rather than simply measuring either hearing protector attenuation or unprotected individual exposure.
The monitoring system includes at least one microphone, housed in the interior of a hearing protective device. Exposure dosage calculation includes periods when the HPD is worn (primary microphone position) and periods when it is not worn (secondary microphone position). When the HPD is worn at the primary position, it measures the noise level with the protective device in effect, and when the HPD is worn at the secondary position, it measures the noise level of the environment without the protective device. This provides an accurate measurement of the actual exposure dosage because invariably workers have their hearing protectors donned for part of the day and removed for the rest of the day.
Methods of measuring noise dose or sound level incident upon the worker's ear canal (“in-the-ear-canal sound level measurements”) are known in the prior art, but are performed with a probe tube microphone. These devices are not suitable for measuring under hearing protectors. First, the tube breaks the seal of the protector, thus compromising the protection. Second, the tube is acoustically transparent at some frequencies, making the system inappropriate for use in high noise areas. For example, U.S. Pat. No. 5,757,930 to Seidemann, describes a system of noise measurement interior to an insert-type hearing protector. This system consists of a microphone mounted at the interior tip of a modified earplug, not mounted in the ear canal. The Seidemann system is designed to measure hearing protector attenuation, not personal noise exposure. Once the earplug is removed (unprotected condition), it no longer measures the noise exposure level in the ear canal. Furthermore, the Seidemann system is not usable with muff-type HPDs.